Back-arc basins are key sites for oceanic lithosphere formation and consumption at convergent plate boundaries,and their formation and subduction processes can be highly variable.The tectonic setting and evolution of ...Back-arc basins are key sites for oceanic lithosphere formation and consumption at convergent plate boundaries,and their formation and subduction processes can be highly variable.The tectonic setting and evolution of the Meso-Tethys Shiquanhe-Jiali ophiolite sub-belt(SJO sub-belt)within BangongNujiang Suture Zone(BNSZ),central xizang,are disputed for the complex rock composition and ages.In this paper,we present geochronology,geochemistry and field observations on the Shiquanhe ophiolite,providing a representative ophiolite example in the western end of SJO.Based on investigation of the petrogenesis and tectonic setting of different rock types,combined with the U-Pb dating,we propose a twostage subduction model for explaining the tectonic evolution of SJO as well as the wither away of a backarc basin.Geochemical and geochronological data indicate that the ca.183 Ma LAN(north of Lameila)gabbros formed in the forearc setting and represent the early-stage subduction of the Bangong MesoTethys.This subduction induced the back-arc spreading recorded in the ca.170 Ma gabbros and lower pillow basalts of PL-SDN(Pagelizanong-Shiquanhe Dam Nan)ophiolitic fragments in the Shiquanhe ophiolite.The basaltic lavas overlying the lower basalts,represented by the ca.168–164 Ma diabasic and boninite dikes have forearc characteristics,and they represent the back-arc basin subduction initiation at a late stage.This work thus recovered the multiple tectonic evolution of SJO sub-belt and emphasise the importance of the back-arc basin subduction in the evolution of ancient oceans.展开更多
High-Mg andesite/diorite(HMA)is useful for identifying subduction-related processes in orogenic belts,including the identification of ophiolites formed in suprasubduction zone(SSZ)environments.The E'rentaolegai hi...High-Mg andesite/diorite(HMA)is useful for identifying subduction-related processes in orogenic belts,including the identification of ophiolites formed in suprasubduction zone(SSZ)environments.The E'rentaolegai high-Mg diorite from the Diyanmiao ophiolite in central Inner Mongolia,North China,has been investigated revealing low-K tholeiitic-calc-alkaline characteristics and have SiO_(2)contents of 53.44-54.92 wt%,MgO contents of 8.44-9.54 wt%,and Mg~#of 54.35-57.60,with variable Fe_(2)O_(3)(7.51-8.61 wt%),Al_(2)O_(3)(11.95-15.09 wt%),and Na_(2)O(3.42-3.94 wt%)contents,low K_(2)O(0.34-0.97 wt%),TiO_(2)(0.35-0.67 wt%),and P2O5(0.12-0.15 wt%)contents,and high Ni(43-193 ppm)and Cr(189-556 ppm)contents.Samples collected have low total rare earth element(REE)contents(30.58-77.80 ppm),with flat or slightly right-dipping REE patterns(La_(N)/Yb_(N)=2.19-3.11)and a lack of pronounced Eu anomalies.The samples are also enriched in large-ion lithophile elements(LILEs,e.g.,K,Rb,Ba,U,and Sr)and depleted in high field strength elements(e.g.,Ta,Nb,Ti,and P).The E'rentaolegai high-Mg diorite has characteristics typical of HMA,and are similar to those of sanukites from the Setouchi Arc in SW Japan.They also display high positiveε_(Nd)(t)values(+6.32 to+7.80),comparable to the values of their host rocks.Petrogenetic analyses suggest that the E'rentaolegai HMA was probably formed by the interaction of partial melts and aqueous fluids from subducted sediments with mantle peridotite.Zircon U-Pb dating reveals that the high-Mg diorite crystallized at 313.6±2.4 Ma,i.e.,late Carboniferous.Combining our data with the temporal and spatial distribution of the Diyanmiao SSZ-type ophiolite,we propose that the eastern Paleo-Asian Ocean had not closed by the late Carboniferous,but intra-oceanic subduction was ongoing.A new model of the initiation of subduction in the eastern PAO during the late Paleozoic.展开更多
Fragments of Proto-Tethyan oceanic lithosphere are well-preserved along the southern belt of the North Qilian suture,and the origin and emplacement of these ophiolites have become subjects of intense debate.In this st...Fragments of Proto-Tethyan oceanic lithosphere are well-preserved along the southern belt of the North Qilian suture,and the origin and emplacement of these ophiolites have become subjects of intense debate.In this study,we integrate field observations,mineralogical and geochemical analyses,zircon U-Pb dating,and isotopic data to investigate the Yanglong ophiolite.The Yanglong ophiolitic rocks are found as tectonic slices resting on the Neoproterozoic sedimentary and volcanic rocks.These rocks are composed of Cambrian serpentinized peridotite,gabbro,dolerite,and rodingite.The spinels in the serpentinized peridotites have variable Cr^(#)values(21,38-46,and 59-61)and display affinity to those in abyssal and forearc peridotites.The dolerites show slight enrichment in Th and have elevated(La/Sm)_(N) ratios(1.19-2.01),indicating a subduction-related geochemical affinity.The Yanglong ophiolitic rocks have positive zirconεHf(t)values(+10.3 to+18.4)and whole-rockεNd(t)values(+5.3 to+6.7)indicating derivation from partial melting of a depleted mantle source.These results,together with the regional geology,collectively suggest that the Yanglong ophiolite was generated in a forearc setting during the Early Cambrian northward intra-oceanic subduction.It was emplaced onto the Central Qilian Block during the subsequent arc-continent collision,no later than the Early Ordovician.展开更多
The Noorabad-Harsin ophiolite is a part of the eastern Mediterranean-Zagros-Oman Tethyan ophiolites. This area is located in the south-southwest of the main Zagros thrust zone. This ophiolite consists of peridotites, ...The Noorabad-Harsin ophiolite is a part of the eastern Mediterranean-Zagros-Oman Tethyan ophiolites. This area is located in the south-southwest of the main Zagros thrust zone. This ophiolite consists of peridotites, serpentinites and pegmatite gabbros as mantle sequence whereas crustal sequence is composed of locally layered gabbros, isotropic gabbros, sheeted dike complex, basaltic to andesitic lavas and sedimentary rocks （radiolarites and Late Cretaceous pelagic limes- tones）. The diabase dikes are enriched in LREE relative to HREE （La（n）/Yb（n）=1.7-3.3）. Also, the andesites are enriched in LREE relative to HREE （La（n）/Yb（n）=3.1-5.37） and the pillow lavas are enriched in LILE （Th（n）/La（n）=2.1） while show a depletion in HFSE （Nb（n）/La（n）=0.07-0.2）. The basaltic-andesitic lavas exhibiting mainly calc-alkaline, with minor island-arc tholeiitic affinities, are characterized by enrichment in LILE and LREE and depletion in HFSE. These geochemical characteristics compared with other Tethyan ophiolites along the Bitlis-Zagros suture zone reveal a supra- subduction zone environment for the genesis of the Noorabad-Harsin ophiolites.展开更多
The recently identified Huashan ophiolitic mélange was considered as the eastern part of the Mianlüe suture in the Qinling orogenic belt. SHRIMP zircon U-Pb geochronology on gabbro from the Huashan ophiolite...The recently identified Huashan ophiolitic mélange was considered as the eastern part of the Mianlüe suture in the Qinling orogenic belt. SHRIMP zircon U-Pb geochronology on gabbro from the Huashan ophiolite and granite intruding basic volcanic rocks indicates crystallization ages of 947±14 Ma and 876±17 Ma respectively. These ages do not support a recently proposed Hercynian Huashan Ocean, but rather favor that a Neoproterozoic suture assemblage （ophiolite） is incorporated into the younger （Phanerozoic） Qinling orogenic belt.展开更多
The subduction of the Bangonghu-Nujiang Meso-Tethys and the collision between the Lhasa and Qiangtang blocks were important events in the growth of the Tibetan crust. However, the timing of collision initiation and cl...The subduction of the Bangonghu-Nujiang Meso-Tethys and the collision between the Lhasa and Qiangtang blocks were important events in the growth of the Tibetan crust. However, the timing of collision initiation and closure timing, as well as nature and structure of the Bangonghu ocean basin, are still poorly constrained. The Lagkor Tso ophiolite, located in the south of Gerze County, Tibet, is one of the most completed ophiolites preserved in the southern side of the Bangonghu- Nujiang suture zone. This study discussed the tectonic evolution of the Bangonghu-Nujiang suture zone as revealed by the Lagkor Tso ophiolite investigated by field investigations, petrology, geochemistry, geochronology and tectonic analysis methods. We present new LA-ICP-MS zircon U-Pb and 39Ar/4~Ar ages for the Lagkor Tso ophiolite, in addition to geochemical and platinum-group element （PGE） data presented for the Lagkor Tso ophiolite in Tibet. It is suggested that the ancient Lagkor Tso oceanic basin split in Middle Jurassic （161.2 ± 2.7 Ma - 165.4 ± 3.5 Ma）, and experienced a second tectonic emplacement during the Early Cretaceous （137.90 ± 6.39 Ma）. The Lagkor Tso ophiolite likely developed in an independent suture zone. The Bangonghu-Nujiang ocean subducted southwards, and the dehydration of the subducting oceanic crust materials caused partial melting of the continental mantle wedge, which formed the second-order expanding center of the obduction dish. This led to inter-arc expansion, followed by the formation of inter-arc and back-arc basins with island arc features, which are represented by ophiolites around the Shiquanhe-Lagkor Tso -Yongzhu region. The tectonic environment presently can be considered to be similar to that of the current Western Pacific, in which a large number of island arc-ocean basin systems are developed.展开更多
The Bayan Gol ophiolite fragment is a portion of the North Tianshan Early Carboniferous ophiolite belt. This ophiolite belt represents a geological record of an Early Carboniferous 'Red Sea type' ocean basin t...The Bayan Gol ophiolite fragment is a portion of the North Tianshan Early Carboniferous ophiolite belt. This ophiolite belt represents a geological record of an Early Carboniferous 'Red Sea type' ocean basin that was developed on the northern margin of the Tianshan Carboniferous-Permian rift system in northwestern China. The late Early Carboniferous Bayan Gol ophiolite suite was emplaced in an Early Carboniferous rift volcano- sedimentary succession of shallow-marine to continental facies (Volcanics Unit). Ophiolitic rocks in the Bayan Gol area comprise ultramafic rocks, gabbros with associated plagiogranite veins, diorite, diabase, pillow basalts and massive lavas. The Early Carboniferous rifting and the opening process of the North Tianshan ocean basin produced mafic magmas in composition of tholeiite and minor amounts of evolved magmas. Compositions of trace elements and Nd, Sr and Pb isotopes reveal the presence of two distinct mantle sources: (1) the Early Carboniferous rift mafic lavas from the Volcanics Unit were generated by a relatively low degree of partial melting of an asthenospheric OIB-type intraplate source; (2) younger (late Early Carboniferous, ~324.8 Ma ago) mafic lavas from the Ophiolite Unit were formed in a relatively depleted MORB-like mantle source, located in the uppermost asthenosphere and then gradually mixed with melts from the asthenospheric OIB-like mantle. A slight interaction between asthenosphere-derived magmas and lithospheric mantle took place during ascent to the surface. Subsequently, the most depleted mafic lavas of the ophiolite assemblage were contaminated by upper-crustal components (seawater or carbonate crust).展开更多
Various combinations of diamond, moissanite, zircon, quartz, corundum, rutile, titanite, almandine garnet, kyanite, and andalusite have been recovered from the Dangqiong peridotites. More than 80 grains of diamond hav...Various combinations of diamond, moissanite, zircon, quartz, corundum, rutile, titanite, almandine garnet, kyanite, and andalusite have been recovered from the Dangqiong peridotites. More than 80 grains of diamond have been recovered, most of which are pale yellow to reddish-orange to colorless. The grains are all 100-200 μm in size and mostly anhedral, but with a range of morphologies including elongated, octahedral and subhedral varieties. Their identification was confirmed by a characteristic shift in the Raman spectra between 1325 cm^-1 and 1333 cm^-1, mostly at 1331.51 cm^-1 or 1326.96 cm^-1. Integration of the mineralogical, petrological and geochemical data for the Dongqiong peridotites suggests a multi-stage formation for this body and similar ophiolites in the Yarlung-Zangbo suture zone. Chromian spinel grains and perhaps small bodies of chromitite crystallized at various depths in the upper mantle, and encapsulated the UHP, highly reduced and crustal minerals. Some oceanic crustal slabs containing the chromian spinel and their inclusion were later trapped in suprasubduction zones（SSZ）, where they were modified by island arc tholeiitic and boninitic magmas, thus changing the chromian spinel compositions and depositing chromitite ores in melt channels.展开更多
The A＇nyemaqen （阿尼玛卿） ophiolite belt along the southern margin of the East Kunlun （昆仑） Mountains marks the suture formed by the closure of paleo-Tethys. The Dur＇ngoi ophiolite in the eastern part of this b...The A＇nyemaqen （阿尼玛卿） ophiolite belt along the southern margin of the East Kunlun （昆仑） Mountains marks the suture formed by the closure of paleo-Tethys. The Dur＇ngoi ophiolite in the eastern part of this belt consists of meta-peridotite, mafic-ultramafic cumulates, sheeted dikes and basaltic lavas. The meta-peridotites consist of dunite, harzburgite, lherzolite, feldspar-bearing lherzolite and garnet-bearing lherzolite and contain residual spinel with Cr# [100×Cr/（Cr＋Al）] ranging from 30 to 57 and Mg# [100×Mg/（Mg＋Fe2＋）] ranging from 50 to 75, indicating an Al- and Mg-rich series. The meta-peridotites have a relatively narrow range of composition with Mg# of 89.2-92.6, Al2O3 contents of （1-4） wt.% and slightly depleted chondrite normalized REE patterns, indicating that they represent relict mantle material that has undergone intermediate to low degrees of partial melting. Garnets in the lherzolite are andradite, enriched in Ca and Fe and depleted in Mg and AI （And=95-97, Pyr=0.3-5, Gro=0-3）, indicating a metamorphic origin. The cumulate rocks mainly consist of dunite, wehrlite, pyroxenite and gabbro. A well-layered gabbro-pyroxenite complex is defined by modal variations in plagioclase and pyroxene. Blocks of garnet-pyroxenite or rodingite are locally present in the meta-peridotites. Garnets in the cumulate rocks are grossular （Gro=69-90, And=9-19, Br=l-12）, also metamorphic origin. The diabase dikes are moderately depleted in LREE [（La/Sm）N=0.5-0.8] and HREE resulting in slightly convex chondrite-normalized patterns with slightly positive Eu anomalies （δEu=1.1-1.3）. The basaltic lavas have REE patterns similar to those of MORB with （La/Sm）N ratios of 0.5-1 and small negative Eu anomalies. They appear to have been derived from a depleted mantle source and to have undergone little or no differentiation during crystallization. SHRIMP U-Pb dating of zircons from the basalts yields 286pb/238U ages of 276-319 Ma （average 308.0±4.9 Ma）. The Dur＇ngoi ophiolite is interpreted as a dismembered fragment of paleo-oceanic crust emplaced during closure of the paleo-Tethyan Ocean basin. Three other suites of oceanic lavas are recognized in the area： island arc volcanic （IAV） rocks, possible back arc basin （BAB） basalts and possible post-collisional volcanic （PCV） and plutonic rocks. The distribution of these rocks suggests north-directed subduction. Opening of the A＇nyemaqen oceanic basin started at least as early as Late Carboniferous （308 Ma） and the basin probably closed during the Early Triassic. The IAV formed in Late Permian （260 Ma）, the BAB in Early-Middle Triassic, and the PCV in Late Triassic. Several large scale, ductile, sinistral strike-slip fault zones, extending hundreds to thousands kilometers, formed along or north of the suture during the Early-Late Triassic, e.g., they are the south margin fault zone of East Kunlun （200-220 Ma）, the Altyn Tagh fault （220-230 Ma）, and the North Qaidam fault zone （240-250 Ma）. These strike-slip faults were probably generated by oblique subduction and closure of the paleo-Tethyan Ocean basin, possibly during exhumation of the subducted plate or uplift of the overriding plate, coincident with post-collisional magmatism.展开更多
The Mianlue suture extends from Derni-Nanping-Pipasi-Kangxian to the Lueyang-Mianxian area, then traverses the Bashan arcuate structure eastward to the Huashan region, and finally to the Qingshuihe area of the souther...The Mianlue suture extends from Derni-Nanping-Pipasi-Kangxian to the Lueyang-Mianxian area, then traverses the Bashan arcuate structure eastward to the Huashan region, and finally to the Qingshuihe area of the southern Dabie Mountains. From east to west, with a length of over 1500 km, the ophiolitic melange associations are distributed discontinuously along the suture. The rock assemblages include ophiolite, island-arc and oceanic island rock series, indicating that there existed a suture zone and a vanished paleo-ocean basin. The Mianliie paleo-ocean basin experienced its main expansion and formation process during the Carboniferous-Permian and closed totally in the Triassic. It belongs to the northern branch of the eastern paleotethys, separated from the northern margin of the Yangtze block under the paleotethys mantle dynamic system.展开更多
Ophiolites are widespread along the Bangong-Nujiang suture zone, northern Tibet. However, it is still debated on the formation ages and tectonic evolution process of these ophiolites. The Zhongcang ophiolite is a typi...Ophiolites are widespread along the Bangong-Nujiang suture zone, northern Tibet. However, it is still debated on the formation ages and tectonic evolution process of these ophiolites. The Zhongcang ophiolite is a typical ophiolite in the western part of the Bangong-Nujiang suture zone. It is composed of serpentinized peridotite, cumulate and isotropic gabbros, massive and pillow basalts, basaltic volcanic breccia, and minor red chert. Zircon SHRIMP Ue Pb dating for the isotropic gabbro yielded weighted mean age of 163.4 ± 1.8 Ma. Positive zircon ε Hf(t) values(+15.0 to +20.2) and mantle-like σ^(18)O values(5.29 ±0.21)% indicate that the isotropic gabbros were derived from a long-term depleted mantle source. The isotropic gabbros have normal mid-ocean ridge basalt(N-MORB) like immobile element patterns with high Mg O, low TiO_2 and moderate rare earth element(REE) abundances, and negative Nb,Ti, Zr and Hf anomalies. Basalts show typical oceanic island basalt(OIB) geochemical features, and they are similar to those of OIB-type rocks of the Early Cretaceous Zhongcang oceanic plateau within the Bangong-Nujiang Ocean. Together with these data, we suggest that the Zhongcang ophiolite was probably formed by the subduction of the Bangong-Nujiang Ocean during the Middle Jurassic. The subduction of the Bangong-Nujiang Tethyan Ocean could begin in the Earlye Middle Jurassic and continue to the Early Cretaceous, and finally continental collision between the Lhasa and Qiangtang terranes at the west Bangong-Nujiang suture zone probably has taken place later than the Early Cretaceous(ca. 110 Ma).展开更多
The ultrahasic rocks in the Mianxian-Lueyang ophiolite belt are mainly harzburgite and dunite, and exhibit two types of REE distribution pattern: (1) LREE depletion, with remarkable Eu enrichment; (2) slight enrichmen...The ultrahasic rocks in the Mianxian-Lueyang ophiolite belt are mainly harzburgite and dunite, and exhibit two types of REE distribution pattern: (1) LREE depletion, with remarkable Eu enrichment; (2) slight enrichment of LREE, with Eu deficiency. Both gabbros (accumulative gabbros) and diabasic dike swarms show a LREE enrichment but a slight Eu anomaly. The ratios of w (Ti) /w (V), w (Th) /w (Ta). w (Th) /w (Yb) and w (Ta) /w (Yb) indicate that the mid-ocean ridge basalts in this region originated from depleted mantle of asthenosphere, implying an association of MORB-type ophiolite and an ancient ocean basin between Qinling and Yangtze plates during Middle Paleozoic-Early Mesozoic era. The island arc volcanic rocks can be divided into tholeiitic and cafe-alkaline associations, which originated from the upper mantle wedge above slab by partial melting.展开更多
The Pulan-Xiangquanhe ophiolite in the western Yarlung Tsangpo suture zone of Tibet is investigated for its geochemistry,geochronology,and tectonic implications in detail.Sensitive high resolution ion micro-probe zirc...The Pulan-Xiangquanhe ophiolite in the western Yarlung Tsangpo suture zone of Tibet is investigated for its geochemistry,geochronology,and tectonic implications in detail.Sensitive high resolution ion micro-probe zircon U-Pb dating reveals that diabases in the ophiolite from the three locations of Xugugab,Mapam Yum Co and La＇nga Co are dated at 122.3±2.5 Ma,118.8±1.8 Ma and 120.5±1.9 Ma,respectively.These early Cretaceous mafic rocks have Na_2O＋K_2O,rare earth element patterns,trace elemental spider diagram and other geochemical fingerprints of typical mature back-arc basin affinity.Therefore,the Pulan-Xiangquanhe ophiolite formed under a mature back-arc basin environment,which was a product of this intra-oceanic subduction system.A suprasubduction system could have existed in the southern margin of Eurasia,which involved both intra-oceanic and continent-ward subductions.Extension dominated the southern margin of the Eurasian continent during the early Cretaceous.展开更多
The Xigaze ophiolite is located in the middle section of the Yarlung Zangbo River ophiolite belt and includes a well-preserved sequence section of seven ophiolite blocks. The relatively complete ophiolitic sequence se...The Xigaze ophiolite is located in the middle section of the Yarlung Zangbo River ophiolite belt and includes a well-preserved sequence section of seven ophiolite blocks. The relatively complete ophiolitic sequence sections are represented by Jiding, Dejixiang, Baigang, and Dazhuqu ophiolites and consist of three-four units. The complete ophiolite sequence in order from the bottom to top consists of mantle peridotite, cumulates, sheeted sill dike swarms, and basic lavas±radiolarian chert. These cumulates are absent in the remaining blocks of Dejixiang and Luqu. The age of radiolaria in the radiolarian chert is Late Jurassic-Cretaceous. The basalt and ultramafic rock of the ophiolite also are overlaid by Tertiary Liuqu conglomerate, which contains numerous pebble components of ophiolite, indicating that the Tethys Ocean began to close at the end of Cretaceous Period. The isotopic data of gabbro, diabase, and albite granite in the Xigaze ophiolite are approximately 126-139 Ma, which indicates that the ophiolite formed in the Early Cretaceous. The K-Ar age of amphibole in garnet amphibolite in the ophiolite melange is 81 Ma, indicating that tectonic ophiolite emplacement occurred at the end of Late Cretaceous. Research in petrology, petrological chemistry, mineralogy, and geochemistry of volcanic rocks and dikes of the Xigaze ophiolite indicate the following characteristics： （1） They are mainly composed of basalt, basaltic andesite, dolerite, and diabase and are characterized by high TiO2 （0.7-1.47%）, low MgO （mostly less than 8%）, and low SiO2 （mostly less than 53%）. （2） The volcanic rocks and dikes of the Xigaze ophiolite show light rare earth element （LREE）-depleted rare earth element （REE） patterns. （3） The spider diagrams of the volcanic rocks and dikes of the Xigaze ophiolite exhibit LILE depletion relative to high-field-strength element （HFSE） patterns with left oblique features. （4） No protogenetic olivine and clinoenstatite was detected. （5） Some dikes show low TiO2 and high MgO, in which a few of Cr-enriched spinels and a very few pseudomorphs of olivine, orthopyroxene can be seen. They show more distinctive affinity as boninitic rock and canbe classified to boninite series rock. The previously mentioned features of the volcanic rocks and dikes in the Xigaze ophiolite implies that these ophiolites formed in a mid-ocean ridge （MOR） in the earlier stage and than forearc extension of subduction initiation occurred once at the later stage of the evolution of the Xigaze ophiolite. The forearc extention caused further melting of the residue-depleted mantle, resulting in the formation of melts with lower TiO2 and higher MgO. These melts formed as dikes and intruded into the oceanic crust formed in the earlier stage, resulting in a close association of mid-ocean ridge basalt and the boninite rock of the Xigaze ophiolite.展开更多
In recent years diamonds and other unusual minerals （carbides, nitrides, metal alloys and native elements） have been recovered from mantle peridotites and chromitites （both high-Cr chromitites and high-Al chromiti...In recent years diamonds and other unusual minerals （carbides, nitrides, metal alloys and native elements） have been recovered from mantle peridotites and chromitites （both high-Cr chromitites and high-Al chromitites） from a number of ophiolites of different ages and tectonic settings. Here we report a similar assemblage of minerals from the Skenderbeu massif of the Mirdita zone ophiolite, west Albania. So far, more than 20 grains of microdiamonds and 30 grains of moissanites （SIC） have been separated from the podiform chromitite. The diamonds are mostly light yellow, transparent, euhedral crystals, 200-300μm across, with a range of morphologies; some are octahedral and cuboctahedron and others are elongate and irregular. Secondary electron images show that some grains have well-developed striations. All the diamond grains have been analyzed and yielded typical Raman spectra with a shift at -1325 cm^-1. The moissanite grains recovered from the Skenderben chromitites are mainly light blue to dark blue, but some are yellow to light yellow. All the analyzed grains have typical Raman spectra with shifts at 766 cm^-1, 787 cm^-1, and 967 cm^-1. The energy spectrums of the moissanites confirm that the grains are composed entirely of silicon and carbon. This investigation expands the occurrence of diamonds and moissanites to Mesozoic ophiolites in the Neo-Tethys. Our new findings suggest that diamonds and moissanites are present, and probably ubiquitous in the oceanic mantle and can provide new perspectives and avenues for research on the origin of ophiolites and podiform chromitites.展开更多
Various combinations of diamond, moissanite, zircon, corundum, rutile and titanitehave been recovered from the Bulqiza chromitites. More than 10 grains of diamond have been recovered, most of which are pale yellow to ...Various combinations of diamond, moissanite, zircon, corundum, rutile and titanitehave been recovered from the Bulqiza chromitites. More than 10 grains of diamond have been recovered, most of which are pale yellow to reddish–orange to colorless. The grains are all 100–300 μm in size and mostly anhedral, but with a range of morphologies including elongated, octahedral and subhedral varieties. Their identification was confirmed by a characteristic shift in the Raman spectra between 1325 cm-1 and 1333 cm-1, mostly at 1331.51 cm-1 or 1326.96 cm-1. This investigation extends the occurrence of diamond and moissanite to the Bulqiza chromitites in the Eastern Mirdita Ophiolite. Integration of the mineralogical, petrological and geochemical data of the Bulqiza chromitites suggests their multi–stage formation. Magnesiochromite grains and perhaps small bodies of chromitite formed at various depths in the upper mantle, and encapsulated the ultra–high pressure, highly reduced and crustal minerals. Some oceanic crustal slabs containing the magnesiochromite and their inclusion were later trapped in suprasubduction zones, where they were modified by tholeiitic and boninitic arc magmas, thus changing the magnesiochromite compositions and depositing chromitite ores in melt channels.展开更多
In this study, plagiogranites in the Diyanmiao ophiolite of the southeastern Central Asian Orogenic Belt （Altaids） were investigated for the first time. The plagiogranites are composed predominantly of albite and qu...In this study, plagiogranites in the Diyanmiao ophiolite of the southeastern Central Asian Orogenic Belt （Altaids） were investigated for the first time. The plagiogranites are composed predominantly of albite and quartz, and occur as irregular intrusive veins in pillow basalts. The plagiogranites have high SiO2 （74.37-76.68wt%） and low A1203 （11.99-13.30wt%）, and intensively high Na20 （4.52-5.49wt%） and low K20 （0.03-0.40wt%） resulting in high Na20/K20 ratios （11.3-183）. These rocks are classified as part of the low-K tholeiitic series. The plagiogranites have low total rare earth element contents （∑REE）（23.62-39.77ppm）, small negative Eu anomalies （JEu=0.44-0.62）, and flat to slightly LREE-depleted chondrite-normalized REE patterns （（La/Yb）N=0.68-0.76）, similar to N-MORB. The plagiogranites are also characterized by Th, U, Zr, and Hf enrichment, and Nb, P, and Ti depletion, have overall flat primitivemantle-normalized trace element patterns. Field and petrological observations and geochemical data suggest that the plagiogranites in the Diyanmiao ophiolite are similar to fractionation-type plagiogranites. Furthermore, the REE patterns of the plagiogranites are similar to those of the gabbros and pillow basalts in the ophiolite. In plots of SREE-SiO2, La-SiO2, and Yb-SiO2, the plagiogranites, pillow basalts, and gabbros show trends typical of crystal fractionation. As such, the plagiogranites are oceanic in origin, formed by crystal fractionation from basaltic magmas derived from depleted mantle, and are part of the Diyanmiao ophiolite. LA-ICP-MS U-Pb dating of zircons from the plagiogranites yielded ages of 328.6±2.1 and 327.1±2.1Ma, indicating an early Carboniferous age for the Diyanmiao ophiolite. These results provide petrological and geochronological evidence for the identification of the Erenhot-Hegenshan oceanic basin and Hegenshan suture of the Paleo-Asian Ocean.展开更多
The exsolutious of diopside and magnetite occur as intergrowth and orient within olivine from the mantle dunite, Luobusa ophiolite, Tibet. The dunite is very fresh with a mineral assemblage of olivine （〉95%） ＋ chr...The exsolutious of diopside and magnetite occur as intergrowth and orient within olivine from the mantle dunite, Luobusa ophiolite, Tibet. The dunite is very fresh with a mineral assemblage of olivine （〉95%） ＋ chromite （1%-4%） ＋ diopside （〈1%）. Two types of olivine are found in thin sections： one （Fo = 94） is coarse-grained, elongated with development of kink bands, wavy extinction and irregular margins; and the other （Fo = 96） is fine-grained and poly-angied. Some of the olivine grains contain minor Ca, Cr and Ni. Besides the exsolutions in olivine, three micron-size inclusions are also discovered. Analyzed through energy dispersive system （EDS） with unitary analytical method, the average compositions of the inclusions are： Na20, 3.12%-3.84%; MgO, 19.51%-23.79%; Al2O3, 9.33%-11.31%; SiO2, 44.89%-46.29%; CaO, 11.46%-12.90%; Cr2O3, 0.74%-2.29%; FeO, 4.26%- 5.27%, which is quite similar to those of amphibole. Diopside is anhedral f＇dling between olivines, or as micro-inclusions oriented in olivines. Chromite appears euhedral distributed between olivines, sometimes with apparent compositional zone. From core to rim of the chromite, Fe content increases and Cr decreases; and A! and Mg drop greatly on the rim. There is always incomplete magnetite zone around the chromite. Compared with the nodular chromite in the same section, the euhedral chromite has higher Fe3O4 and lower MgCr2O4 and MgAI2O4 end member contents, which means it formed under higher oxygen fugacity environment. With a geothermometer estimation, the equilibrium crystalline temperature is 820℃-960℃ for olivine and nodular chromite, 630℃-770℃ for olivine and euhedral chromite, and 350℃-550℃ for olivine and exsoluted magnetite, showing that the exsolutions occurred late at low temperature. Thus we propose that previously depleted mantle harzburgite reacted with the melt containing Na, Al and Ca, and produced an olivine solid solution added with Na^＋, Al^3＋, Ca^2＋, Fe^3＋, Cr^3＋. With temperature decreasing, the olivine solid solution decomposed; and Fe^3＋, Cr^3＋ diffused into magnetite and Ca^2＋ and Na^＋ into clinopyroxene, both of which formed intergrowth textures. A few Fe^3＋ and Cr^3＋ entered interstitial chromite. Through later tectonism, the peridotite recrystallized and formed deformational coarse grained olivine, fine grained and poly-angled olivine, and euhedral grained chromite. Due to the fast cooling rate of the rock or rapid tectonic emplacement, the exsolution textures in olivine and compositional zones of chromite are preserved.展开更多
基金supported by the Guangdong Basic and Applied Basic Research Foundation,China(Grant Nos.2024A1515010439,2025A1515010724)National Nature Science Foundation of China(Grant Nos.41972049,41472054,42072229,41977231)+4 种基金Young Innovative Talent Project of Department of Education of Guangdong Province(Natural ScienceGrant No.2022KQNCX184)Natural Research Project of Guangdong Polytechnic of Industry&Commerce(Grant No.2022-ZKT-01)China State Scholarship Fund of visiting scholar(Grant No.20170638507)High-level Talent Special Support Program of Guangdong Polytechnic of Industry&Commerce(Grant No.2023-gc-03).
文摘Back-arc basins are key sites for oceanic lithosphere formation and consumption at convergent plate boundaries,and their formation and subduction processes can be highly variable.The tectonic setting and evolution of the Meso-Tethys Shiquanhe-Jiali ophiolite sub-belt(SJO sub-belt)within BangongNujiang Suture Zone(BNSZ),central xizang,are disputed for the complex rock composition and ages.In this paper,we present geochronology,geochemistry and field observations on the Shiquanhe ophiolite,providing a representative ophiolite example in the western end of SJO.Based on investigation of the petrogenesis and tectonic setting of different rock types,combined with the U-Pb dating,we propose a twostage subduction model for explaining the tectonic evolution of SJO as well as the wither away of a backarc basin.Geochemical and geochronological data indicate that the ca.183 Ma LAN(north of Lameila)gabbros formed in the forearc setting and represent the early-stage subduction of the Bangong MesoTethys.This subduction induced the back-arc spreading recorded in the ca.170 Ma gabbros and lower pillow basalts of PL-SDN(Pagelizanong-Shiquanhe Dam Nan)ophiolitic fragments in the Shiquanhe ophiolite.The basaltic lavas overlying the lower basalts,represented by the ca.168–164 Ma diabasic and boninite dikes have forearc characteristics,and they represent the back-arc basin subduction initiation at a late stage.This work thus recovered the multiple tectonic evolution of SJO sub-belt and emphasise the importance of the back-arc basin subduction in the evolution of ancient oceans.
基金supported by the Ministry of Science and Technology of the People’s Republic of China(Grant No.2022FY101704)the National Natural Science Foundation of China(Grant No.41972061)+1 种基金Science and Technology Innovation Team of Hebei GEO University(30801071)the China Geological Survey(Grant Nos.1212011120701 and 1212011120711)。
文摘High-Mg andesite/diorite(HMA)is useful for identifying subduction-related processes in orogenic belts,including the identification of ophiolites formed in suprasubduction zone(SSZ)environments.The E'rentaolegai high-Mg diorite from the Diyanmiao ophiolite in central Inner Mongolia,North China,has been investigated revealing low-K tholeiitic-calc-alkaline characteristics and have SiO_(2)contents of 53.44-54.92 wt%,MgO contents of 8.44-9.54 wt%,and Mg~#of 54.35-57.60,with variable Fe_(2)O_(3)(7.51-8.61 wt%),Al_(2)O_(3)(11.95-15.09 wt%),and Na_(2)O(3.42-3.94 wt%)contents,low K_(2)O(0.34-0.97 wt%),TiO_(2)(0.35-0.67 wt%),and P2O5(0.12-0.15 wt%)contents,and high Ni(43-193 ppm)and Cr(189-556 ppm)contents.Samples collected have low total rare earth element(REE)contents(30.58-77.80 ppm),with flat or slightly right-dipping REE patterns(La_(N)/Yb_(N)=2.19-3.11)and a lack of pronounced Eu anomalies.The samples are also enriched in large-ion lithophile elements(LILEs,e.g.,K,Rb,Ba,U,and Sr)and depleted in high field strength elements(e.g.,Ta,Nb,Ti,and P).The E'rentaolegai high-Mg diorite has characteristics typical of HMA,and are similar to those of sanukites from the Setouchi Arc in SW Japan.They also display high positiveε_(Nd)(t)values(+6.32 to+7.80),comparable to the values of their host rocks.Petrogenetic analyses suggest that the E'rentaolegai HMA was probably formed by the interaction of partial melts and aqueous fluids from subducted sediments with mantle peridotite.Zircon U-Pb dating reveals that the high-Mg diorite crystallized at 313.6±2.4 Ma,i.e.,late Carboniferous.Combining our data with the temporal and spatial distribution of the Diyanmiao SSZ-type ophiolite,we propose that the eastern Paleo-Asian Ocean had not closed by the late Carboniferous,but intra-oceanic subduction was ongoing.A new model of the initiation of subduction in the eastern PAO during the late Paleozoic.
基金the National Natural Science Foundation of China(Grant Nos.42230308,42072266,42102032,42362006)Bureau of Geological Exploration and Development of Qinghai Province(Grant Nos.[2023]33,[2022]32)China Geological Survey(Grant Nos.DD20221649,DD20242935,12120114079701).
文摘Fragments of Proto-Tethyan oceanic lithosphere are well-preserved along the southern belt of the North Qilian suture,and the origin and emplacement of these ophiolites have become subjects of intense debate.In this study,we integrate field observations,mineralogical and geochemical analyses,zircon U-Pb dating,and isotopic data to investigate the Yanglong ophiolite.The Yanglong ophiolitic rocks are found as tectonic slices resting on the Neoproterozoic sedimentary and volcanic rocks.These rocks are composed of Cambrian serpentinized peridotite,gabbro,dolerite,and rodingite.The spinels in the serpentinized peridotites have variable Cr^(#)values(21,38-46,and 59-61)and display affinity to those in abyssal and forearc peridotites.The dolerites show slight enrichment in Th and have elevated(La/Sm)_(N) ratios(1.19-2.01),indicating a subduction-related geochemical affinity.The Yanglong ophiolitic rocks have positive zirconεHf(t)values(+10.3 to+18.4)and whole-rockεNd(t)values(+5.3 to+6.7)indicating derivation from partial melting of a depleted mantle source.These results,together with the regional geology,collectively suggest that the Yanglong ophiolite was generated in a forearc setting during the Early Cambrian northward intra-oceanic subduction.It was emplaced onto the Central Qilian Block during the subsequent arc-continent collision,no later than the Early Ordovician.
文摘The Noorabad-Harsin ophiolite is a part of the eastern Mediterranean-Zagros-Oman Tethyan ophiolites. This area is located in the south-southwest of the main Zagros thrust zone. This ophiolite consists of peridotites, serpentinites and pegmatite gabbros as mantle sequence whereas crustal sequence is composed of locally layered gabbros, isotropic gabbros, sheeted dike complex, basaltic to andesitic lavas and sedimentary rocks （radiolarites and Late Cretaceous pelagic limes- tones）. The diabase dikes are enriched in LREE relative to HREE （La（n）/Yb（n）=1.7-3.3）. Also, the andesites are enriched in LREE relative to HREE （La（n）/Yb（n）=3.1-5.37） and the pillow lavas are enriched in LILE （Th（n）/La（n）=2.1） while show a depletion in HFSE （Nb（n）/La（n）=0.07-0.2）. The basaltic-andesitic lavas exhibiting mainly calc-alkaline, with minor island-arc tholeiitic affinities, are characterized by enrichment in LILE and LREE and depletion in HFSE. These geochemical characteristics compared with other Tethyan ophiolites along the Bitlis-Zagros suture zone reveal a supra- subduction zone environment for the genesis of the Noorabad-Harsin ophiolites.
基金the National Natural Science Foundation of China （Grant No. 40234045, 40473030）.
文摘The recently identified Huashan ophiolitic mélange was considered as the eastern part of the Mianlüe suture in the Qinling orogenic belt. SHRIMP zircon U-Pb geochronology on gabbro from the Huashan ophiolite and granite intruding basic volcanic rocks indicates crystallization ages of 947±14 Ma and 876±17 Ma respectively. These ages do not support a recently proposed Hercynian Huashan Ocean, but rather favor that a Neoproterozoic suture assemblage （ophiolite） is incorporated into the younger （Phanerozoic） Qinling orogenic belt.
基金supported by the National Nature Science Foundation of China (grant No.41372208)China Geological Survey (grant No.1212011221105 and 1212011121259)
文摘The subduction of the Bangonghu-Nujiang Meso-Tethys and the collision between the Lhasa and Qiangtang blocks were important events in the growth of the Tibetan crust. However, the timing of collision initiation and closure timing, as well as nature and structure of the Bangonghu ocean basin, are still poorly constrained. The Lagkor Tso ophiolite, located in the south of Gerze County, Tibet, is one of the most completed ophiolites preserved in the southern side of the Bangonghu- Nujiang suture zone. This study discussed the tectonic evolution of the Bangonghu-Nujiang suture zone as revealed by the Lagkor Tso ophiolite investigated by field investigations, petrology, geochemistry, geochronology and tectonic analysis methods. We present new LA-ICP-MS zircon U-Pb and 39Ar/4~Ar ages for the Lagkor Tso ophiolite, in addition to geochemical and platinum-group element （PGE） data presented for the Lagkor Tso ophiolite in Tibet. It is suggested that the ancient Lagkor Tso oceanic basin split in Middle Jurassic （161.2 ± 2.7 Ma - 165.4 ± 3.5 Ma）, and experienced a second tectonic emplacement during the Early Cretaceous （137.90 ± 6.39 Ma）. The Lagkor Tso ophiolite likely developed in an independent suture zone. The Bangonghu-Nujiang ocean subducted southwards, and the dehydration of the subducting oceanic crust materials caused partial melting of the continental mantle wedge, which formed the second-order expanding center of the obduction dish. This led to inter-arc expansion, followed by the formation of inter-arc and back-arc basins with island arc features, which are represented by ophiolites around the Shiquanhe-Lagkor Tso -Yongzhu region. The tectonic environment presently can be considered to be similar to that of the current Western Pacific, in which a large number of island arc-ocean basin systems are developed.
基金the Land and Resources Survey Project of China(Grant Nos.200113000022,200313000063) the National Namral Science Foundation of China fGrant No.40472044).
文摘The Bayan Gol ophiolite fragment is a portion of the North Tianshan Early Carboniferous ophiolite belt. This ophiolite belt represents a geological record of an Early Carboniferous 'Red Sea type' ocean basin that was developed on the northern margin of the Tianshan Carboniferous-Permian rift system in northwestern China. The late Early Carboniferous Bayan Gol ophiolite suite was emplaced in an Early Carboniferous rift volcano- sedimentary succession of shallow-marine to continental facies (Volcanics Unit). Ophiolitic rocks in the Bayan Gol area comprise ultramafic rocks, gabbros with associated plagiogranite veins, diorite, diabase, pillow basalts and massive lavas. The Early Carboniferous rifting and the opening process of the North Tianshan ocean basin produced mafic magmas in composition of tholeiite and minor amounts of evolved magmas. Compositions of trace elements and Nd, Sr and Pb isotopes reveal the presence of two distinct mantle sources: (1) the Early Carboniferous rift mafic lavas from the Volcanics Unit were generated by a relatively low degree of partial melting of an asthenospheric OIB-type intraplate source; (2) younger (late Early Carboniferous, ~324.8 Ma ago) mafic lavas from the Ophiolite Unit were formed in a relatively depleted MORB-like mantle source, located in the uppermost asthenosphere and then gradually mixed with melts from the asthenospheric OIB-like mantle. A slight interaction between asthenosphere-derived magmas and lithospheric mantle took place during ascent to the surface. Subsequently, the most depleted mafic lavas of the ophiolite assemblage were contaminated by upper-crustal components (seawater or carbonate crust).
基金funded by grants from Sinoprobe-05-02 from the Ministry of Science and Technology of China,the NSF China(Nos.41502062,40930313,40921001,41202036)the China Geological Survey(Nos.12120115027201,12120114061801,2014DFR2127C)from the Institute of Geology,Chinese Academy of Geological Sciences(J1526)
文摘Various combinations of diamond, moissanite, zircon, quartz, corundum, rutile, titanite, almandine garnet, kyanite, and andalusite have been recovered from the Dangqiong peridotites. More than 80 grains of diamond have been recovered, most of which are pale yellow to reddish-orange to colorless. The grains are all 100-200 μm in size and mostly anhedral, but with a range of morphologies including elongated, octahedral and subhedral varieties. Their identification was confirmed by a characteristic shift in the Raman spectra between 1325 cm^-1 and 1333 cm^-1, mostly at 1331.51 cm^-1 or 1326.96 cm^-1. Integration of the mineralogical, petrological and geochemical data for the Dongqiong peridotites suggests a multi-stage formation for this body and similar ophiolites in the Yarlung-Zangbo suture zone. Chromian spinel grains and perhaps small bodies of chromitite crystallized at various depths in the upper mantle, and encapsulated the UHP, highly reduced and crustal minerals. Some oceanic crustal slabs containing the chromian spinel and their inclusion were later trapped in suprasubduction zones（SSZ）, where they were modified by island arc tholeiitic and boninitic magmas, thus changing the chromian spinel compositions and depositing chromitite ores in melt channels.
基金supported by China Geological Survey (Nos. 1212010918013 and 1212010610105)
文摘The A＇nyemaqen （阿尼玛卿） ophiolite belt along the southern margin of the East Kunlun （昆仑） Mountains marks the suture formed by the closure of paleo-Tethys. The Dur＇ngoi ophiolite in the eastern part of this belt consists of meta-peridotite, mafic-ultramafic cumulates, sheeted dikes and basaltic lavas. The meta-peridotites consist of dunite, harzburgite, lherzolite, feldspar-bearing lherzolite and garnet-bearing lherzolite and contain residual spinel with Cr# [100×Cr/（Cr＋Al）] ranging from 30 to 57 and Mg# [100×Mg/（Mg＋Fe2＋）] ranging from 50 to 75, indicating an Al- and Mg-rich series. The meta-peridotites have a relatively narrow range of composition with Mg# of 89.2-92.6, Al2O3 contents of （1-4） wt.% and slightly depleted chondrite normalized REE patterns, indicating that they represent relict mantle material that has undergone intermediate to low degrees of partial melting. Garnets in the lherzolite are andradite, enriched in Ca and Fe and depleted in Mg and AI （And=95-97, Pyr=0.3-5, Gro=0-3）, indicating a metamorphic origin. The cumulate rocks mainly consist of dunite, wehrlite, pyroxenite and gabbro. A well-layered gabbro-pyroxenite complex is defined by modal variations in plagioclase and pyroxene. Blocks of garnet-pyroxenite or rodingite are locally present in the meta-peridotites. Garnets in the cumulate rocks are grossular （Gro=69-90, And=9-19, Br=l-12）, also metamorphic origin. The diabase dikes are moderately depleted in LREE [（La/Sm）N=0.5-0.8] and HREE resulting in slightly convex chondrite-normalized patterns with slightly positive Eu anomalies （δEu=1.1-1.3）. The basaltic lavas have REE patterns similar to those of MORB with （La/Sm）N ratios of 0.5-1 and small negative Eu anomalies. They appear to have been derived from a depleted mantle source and to have undergone little or no differentiation during crystallization. SHRIMP U-Pb dating of zircons from the basalts yields 286pb/238U ages of 276-319 Ma （average 308.0±4.9 Ma）. The Dur＇ngoi ophiolite is interpreted as a dismembered fragment of paleo-oceanic crust emplaced during closure of the paleo-Tethyan Ocean basin. Three other suites of oceanic lavas are recognized in the area： island arc volcanic （IAV） rocks, possible back arc basin （BAB） basalts and possible post-collisional volcanic （PCV） and plutonic rocks. The distribution of these rocks suggests north-directed subduction. Opening of the A＇nyemaqen oceanic basin started at least as early as Late Carboniferous （308 Ma） and the basin probably closed during the Early Triassic. The IAV formed in Late Permian （260 Ma）, the BAB in Early-Middle Triassic, and the PCV in Late Triassic. Several large scale, ductile, sinistral strike-slip fault zones, extending hundreds to thousands kilometers, formed along or north of the suture during the Early-Late Triassic, e.g., they are the south margin fault zone of East Kunlun （200-220 Ma）, the Altyn Tagh fault （220-230 Ma）, and the North Qaidam fault zone （240-250 Ma）. These strike-slip faults were probably generated by oblique subduction and closure of the paleo-Tethyan Ocean basin, possibly during exhumation of the subducted plate or uplift of the overriding plate, coincident with post-collisional magmatism.
基金the National Nature Science Foundation of China(Grant Nos.40234041, 49732080) the Teaching and Research Award Program for Outstanding YoungTeachers in HigherLearning Institutions of the Ministry of Education of China.
文摘The Mianlue suture extends from Derni-Nanping-Pipasi-Kangxian to the Lueyang-Mianxian area, then traverses the Bashan arcuate structure eastward to the Huashan region, and finally to the Qingshuihe area of the southern Dabie Mountains. From east to west, with a length of over 1500 km, the ophiolitic melange associations are distributed discontinuously along the suture. The rock assemblages include ophiolite, island-arc and oceanic island rock series, indicating that there existed a suture zone and a vanished paleo-ocean basin. The Mianliie paleo-ocean basin experienced its main expansion and formation process during the Carboniferous-Permian and closed totally in the Triassic. It belongs to the northern branch of the eastern paleotethys, separated from the northern margin of the Yangtze block under the paleotethys mantle dynamic system.
基金supported by National Natural Science Foundation of China (Grant Nos. 41522204, 91755103 and 41502216)the Ministry of Science and Technology of China(2016YFC0600304)+1 种基金CAGS Research Fund (Grant No. YYWF201704)Chinese Geological Survey Project (Grant Nos. DD20160123-05 and DD20160345)
文摘Ophiolites are widespread along the Bangong-Nujiang suture zone, northern Tibet. However, it is still debated on the formation ages and tectonic evolution process of these ophiolites. The Zhongcang ophiolite is a typical ophiolite in the western part of the Bangong-Nujiang suture zone. It is composed of serpentinized peridotite, cumulate and isotropic gabbros, massive and pillow basalts, basaltic volcanic breccia, and minor red chert. Zircon SHRIMP Ue Pb dating for the isotropic gabbro yielded weighted mean age of 163.4 ± 1.8 Ma. Positive zircon ε Hf(t) values(+15.0 to +20.2) and mantle-like σ^(18)O values(5.29 ±0.21)% indicate that the isotropic gabbros were derived from a long-term depleted mantle source. The isotropic gabbros have normal mid-ocean ridge basalt(N-MORB) like immobile element patterns with high Mg O, low TiO_2 and moderate rare earth element(REE) abundances, and negative Nb,Ti, Zr and Hf anomalies. Basalts show typical oceanic island basalt(OIB) geochemical features, and they are similar to those of OIB-type rocks of the Early Cretaceous Zhongcang oceanic plateau within the Bangong-Nujiang Ocean. Together with these data, we suggest that the Zhongcang ophiolite was probably formed by the subduction of the Bangong-Nujiang Ocean during the Middle Jurassic. The subduction of the Bangong-Nujiang Tethyan Ocean could begin in the Earlye Middle Jurassic and continue to the Early Cretaceous, and finally continental collision between the Lhasa and Qiangtang terranes at the west Bangong-Nujiang suture zone probably has taken place later than the Early Cretaceous(ca. 110 Ma).
文摘The ultrahasic rocks in the Mianxian-Lueyang ophiolite belt are mainly harzburgite and dunite, and exhibit two types of REE distribution pattern: (1) LREE depletion, with remarkable Eu enrichment; (2) slight enrichment of LREE, with Eu deficiency. Both gabbros (accumulative gabbros) and diabasic dike swarms show a LREE enrichment but a slight Eu anomaly. The ratios of w (Ti) /w (V), w (Th) /w (Ta). w (Th) /w (Yb) and w (Ta) /w (Yb) indicate that the mid-ocean ridge basalts in this region originated from depleted mantle of asthenosphere, implying an association of MORB-type ophiolite and an ancient ocean basin between Qinling and Yangtze plates during Middle Paleozoic-Early Mesozoic era. The island arc volcanic rocks can be divided into tholeiitic and cafe-alkaline associations, which originated from the upper mantle wedge above slab by partial melting.
基金supported by the National Natural Science Foundation of China(Grant 40534019)the Major Direction Program of Innovation Engineering of CAS(Grant KZCX2-SW-117-5)
文摘The Pulan-Xiangquanhe ophiolite in the western Yarlung Tsangpo suture zone of Tibet is investigated for its geochemistry,geochronology,and tectonic implications in detail.Sensitive high resolution ion micro-probe zircon U-Pb dating reveals that diabases in the ophiolite from the three locations of Xugugab,Mapam Yum Co and La＇nga Co are dated at 122.3±2.5 Ma,118.8±1.8 Ma and 120.5±1.9 Ma,respectively.These early Cretaceous mafic rocks have Na_2O＋K_2O,rare earth element patterns,trace elemental spider diagram and other geochemical fingerprints of typical mature back-arc basin affinity.Therefore,the Pulan-Xiangquanhe ophiolite formed under a mature back-arc basin environment,which was a product of this intra-oceanic subduction system.A suprasubduction system could have existed in the southern margin of Eurasia,which involved both intra-oceanic and continent-ward subductions.Extension dominated the southern margin of the Eurasian continent during the early Cretaceous.
基金jointly supported by the Geological Survey Project of Chinese (Grant No.1212010911070)National Science Foundation of China (Grant No.41072167)Institute of Geology, Chinese Academy of Geological Sciences (Grant No. J1120)
文摘The Xigaze ophiolite is located in the middle section of the Yarlung Zangbo River ophiolite belt and includes a well-preserved sequence section of seven ophiolite blocks. The relatively complete ophiolitic sequence sections are represented by Jiding, Dejixiang, Baigang, and Dazhuqu ophiolites and consist of three-four units. The complete ophiolite sequence in order from the bottom to top consists of mantle peridotite, cumulates, sheeted sill dike swarms, and basic lavas±radiolarian chert. These cumulates are absent in the remaining blocks of Dejixiang and Luqu. The age of radiolaria in the radiolarian chert is Late Jurassic-Cretaceous. The basalt and ultramafic rock of the ophiolite also are overlaid by Tertiary Liuqu conglomerate, which contains numerous pebble components of ophiolite, indicating that the Tethys Ocean began to close at the end of Cretaceous Period. The isotopic data of gabbro, diabase, and albite granite in the Xigaze ophiolite are approximately 126-139 Ma, which indicates that the ophiolite formed in the Early Cretaceous. The K-Ar age of amphibole in garnet amphibolite in the ophiolite melange is 81 Ma, indicating that tectonic ophiolite emplacement occurred at the end of Late Cretaceous. Research in petrology, petrological chemistry, mineralogy, and geochemistry of volcanic rocks and dikes of the Xigaze ophiolite indicate the following characteristics： （1） They are mainly composed of basalt, basaltic andesite, dolerite, and diabase and are characterized by high TiO2 （0.7-1.47%）, low MgO （mostly less than 8%）, and low SiO2 （mostly less than 53%）. （2） The volcanic rocks and dikes of the Xigaze ophiolite show light rare earth element （LREE）-depleted rare earth element （REE） patterns. （3） The spider diagrams of the volcanic rocks and dikes of the Xigaze ophiolite exhibit LILE depletion relative to high-field-strength element （HFSE） patterns with left oblique features. （4） No protogenetic olivine and clinoenstatite was detected. （5） Some dikes show low TiO2 and high MgO, in which a few of Cr-enriched spinels and a very few pseudomorphs of olivine, orthopyroxene can be seen. They show more distinctive affinity as boninitic rock and canbe classified to boninite series rock. The previously mentioned features of the volcanic rocks and dikes in the Xigaze ophiolite implies that these ophiolites formed in a mid-ocean ridge （MOR） in the earlier stage and than forearc extension of subduction initiation occurred once at the later stage of the evolution of the Xigaze ophiolite. The forearc extention caused further melting of the residue-depleted mantle, resulting in the formation of melts with lower TiO2 and higher MgO. These melts formed as dikes and intruded into the oceanic crust formed in the earlier stage, resulting in a close association of mid-ocean ridge basalt and the boninite rock of the Xigaze ophiolite.
基金funded by grants from the International Geoscicence Programme (IGCP 649,2015-2020)the National Natural Science Foundation of China(41541017,41641015)+2 种基金the Ministry of Science and Technology of China(2014DFR21270)China Geological Survey(12120115026801,12120115027201, 201511022,DD20160023-01)the Fund from the State Key Laboratory of Continental Tectonics and Dynamics (Z1301-a20 and K201502)
文摘In recent years diamonds and other unusual minerals （carbides, nitrides, metal alloys and native elements） have been recovered from mantle peridotites and chromitites （both high-Cr chromitites and high-Al chromitites） from a number of ophiolites of different ages and tectonic settings. Here we report a similar assemblage of minerals from the Skenderbeu massif of the Mirdita zone ophiolite, west Albania. So far, more than 20 grains of microdiamonds and 30 grains of moissanites （SIC） have been separated from the podiform chromitite. The diamonds are mostly light yellow, transparent, euhedral crystals, 200-300μm across, with a range of morphologies; some are octahedral and cuboctahedron and others are elongate and irregular. Secondary electron images show that some grains have well-developed striations. All the diamond grains have been analyzed and yielded typical Raman spectra with a shift at -1325 cm^-1. The moissanite grains recovered from the Skenderben chromitites are mainly light blue to dark blue, but some are yellow to light yellow. All the analyzed grains have typical Raman spectra with shifts at 766 cm^-1, 787 cm^-1, and 967 cm^-1. The energy spectrums of the moissanites confirm that the grains are composed entirely of silicon and carbon. This investigation expands the occurrence of diamonds and moissanites to Mesozoic ophiolites in the Neo-Tethys. Our new findings suggest that diamonds and moissanites are present, and probably ubiquitous in the oceanic mantle and can provide new perspectives and avenues for research on the origin of ophiolites and podiform chromitites.
基金funded by grants from the NSF China(Nos.41502062,41672046,41541017,41641015)the China Geological Survey(Nos.DD20160023-01,201511022)+2 种基金National Key Research and Development Project of China(2016YFC0600310)from the Institute of Geology,Chinese Academy of Geological Sciences(J1526)IGCP–649
文摘Various combinations of diamond, moissanite, zircon, corundum, rutile and titanitehave been recovered from the Bulqiza chromitites. More than 10 grains of diamond have been recovered, most of which are pale yellow to reddish–orange to colorless. The grains are all 100–300 μm in size and mostly anhedral, but with a range of morphologies including elongated, octahedral and subhedral varieties. Their identification was confirmed by a characteristic shift in the Raman spectra between 1325 cm-1 and 1333 cm-1, mostly at 1331.51 cm-1 or 1326.96 cm-1. This investigation extends the occurrence of diamond and moissanite to the Bulqiza chromitites in the Eastern Mirdita Ophiolite. Integration of the mineralogical, petrological and geochemical data of the Bulqiza chromitites suggests their multi–stage formation. Magnesiochromite grains and perhaps small bodies of chromitite formed at various depths in the upper mantle, and encapsulated the ultra–high pressure, highly reduced and crustal minerals. Some oceanic crustal slabs containing the magnesiochromite and their inclusion were later trapped in suprasubduction zones, where they were modified by tholeiitic and boninitic arc magmas, thus changing the magnesiochromite compositions and depositing chromitite ores in melt channels.
基金financially supported by the National Natural Science Foundation of China(41502211)the China Geological Survey(1212011120701,1212011120711,12120114064201,DD20160041)the Research Fund for the Doctoral Program of Hebei GEO University(BQ2017052)
文摘In this study, plagiogranites in the Diyanmiao ophiolite of the southeastern Central Asian Orogenic Belt （Altaids） were investigated for the first time. The plagiogranites are composed predominantly of albite and quartz, and occur as irregular intrusive veins in pillow basalts. The plagiogranites have high SiO2 （74.37-76.68wt%） and low A1203 （11.99-13.30wt%）, and intensively high Na20 （4.52-5.49wt%） and low K20 （0.03-0.40wt%） resulting in high Na20/K20 ratios （11.3-183）. These rocks are classified as part of the low-K tholeiitic series. The plagiogranites have low total rare earth element contents （∑REE）（23.62-39.77ppm）, small negative Eu anomalies （JEu=0.44-0.62）, and flat to slightly LREE-depleted chondrite-normalized REE patterns （（La/Yb）N=0.68-0.76）, similar to N-MORB. The plagiogranites are also characterized by Th, U, Zr, and Hf enrichment, and Nb, P, and Ti depletion, have overall flat primitivemantle-normalized trace element patterns. Field and petrological observations and geochemical data suggest that the plagiogranites in the Diyanmiao ophiolite are similar to fractionation-type plagiogranites. Furthermore, the REE patterns of the plagiogranites are similar to those of the gabbros and pillow basalts in the ophiolite. In plots of SREE-SiO2, La-SiO2, and Yb-SiO2, the plagiogranites, pillow basalts, and gabbros show trends typical of crystal fractionation. As such, the plagiogranites are oceanic in origin, formed by crystal fractionation from basaltic magmas derived from depleted mantle, and are part of the Diyanmiao ophiolite. LA-ICP-MS U-Pb dating of zircons from the plagiogranites yielded ages of 328.6±2.1 and 327.1±2.1Ma, indicating an early Carboniferous age for the Diyanmiao ophiolite. These results provide petrological and geochronological evidence for the identification of the Erenhot-Hegenshan oceanic basin and Hegenshan suture of the Paleo-Asian Ocean.
文摘The exsolutious of diopside and magnetite occur as intergrowth and orient within olivine from the mantle dunite, Luobusa ophiolite, Tibet. The dunite is very fresh with a mineral assemblage of olivine （〉95%） ＋ chromite （1%-4%） ＋ diopside （〈1%）. Two types of olivine are found in thin sections： one （Fo = 94） is coarse-grained, elongated with development of kink bands, wavy extinction and irregular margins; and the other （Fo = 96） is fine-grained and poly-angied. Some of the olivine grains contain minor Ca, Cr and Ni. Besides the exsolutions in olivine, three micron-size inclusions are also discovered. Analyzed through energy dispersive system （EDS） with unitary analytical method, the average compositions of the inclusions are： Na20, 3.12%-3.84%; MgO, 19.51%-23.79%; Al2O3, 9.33%-11.31%; SiO2, 44.89%-46.29%; CaO, 11.46%-12.90%; Cr2O3, 0.74%-2.29%; FeO, 4.26%- 5.27%, which is quite similar to those of amphibole. Diopside is anhedral f＇dling between olivines, or as micro-inclusions oriented in olivines. Chromite appears euhedral distributed between olivines, sometimes with apparent compositional zone. From core to rim of the chromite, Fe content increases and Cr decreases; and A! and Mg drop greatly on the rim. There is always incomplete magnetite zone around the chromite. Compared with the nodular chromite in the same section, the euhedral chromite has higher Fe3O4 and lower MgCr2O4 and MgAI2O4 end member contents, which means it formed under higher oxygen fugacity environment. With a geothermometer estimation, the equilibrium crystalline temperature is 820℃-960℃ for olivine and nodular chromite, 630℃-770℃ for olivine and euhedral chromite, and 350℃-550℃ for olivine and exsoluted magnetite, showing that the exsolutions occurred late at low temperature. Thus we propose that previously depleted mantle harzburgite reacted with the melt containing Na, Al and Ca, and produced an olivine solid solution added with Na^＋, Al^3＋, Ca^2＋, Fe^3＋, Cr^3＋. With temperature decreasing, the olivine solid solution decomposed; and Fe^3＋, Cr^3＋ diffused into magnetite and Ca^2＋ and Na^＋ into clinopyroxene, both of which formed intergrowth textures. A few Fe^3＋ and Cr^3＋ entered interstitial chromite. Through later tectonism, the peridotite recrystallized and formed deformational coarse grained olivine, fine grained and poly-angled olivine, and euhedral grained chromite. Due to the fast cooling rate of the rock or rapid tectonic emplacement, the exsolution textures in olivine and compositional zones of chromite are preserved.
